H5M File Backends

H5M File Backends#

cad_to_dagmc supports two backends for writing DAGMC h5m files: h5py (default) and pymoab.

h5py Backend (Default)#

The h5py backend writes h5m files using the h5py library directly. This is the default because it doesn’t require MOAB to be installed.

model.export_dagmc_h5m_file(
    filename="dagmc.h5m",
    h5m_backend="h5py",  # Default
)

Advantages:

No MOAB/pymoab installation required
Available via simple pip install cad_to_dagmc
Produces DAGMC-compatible h5m files

When to use:

When you can’t or don’t want to install MOAB
For most standard use cases
When deploying in environments without MOAB

pymoab Backend#

The pymoab backend uses the official MOAB library to write h5m files.

model.export_dagmc_h5m_file(
    filename="dagmc.h5m",
    h5m_backend="pymoab",
)

Advantages:

Uses the official MOAB library
May have better compatibility with some DAGMC tools

When to use:

When you need guaranteed MOAB compatibility
When working with legacy DAGMC workflows
When pymoab is already installed

Installing pymoab#

pymoab/MOAB must be installed separately (not available on PyPI):

Option 1: Conda (easiest)

conda install -c conda-forge moab

Option 2: Extra index

pip install --extra-index-url https://shimwell.github.io/wheels moab

Option 3: From source

pip install git+https://bitbucket.org/fathomteam/moab/

Error Handling#

If you request pymoab but it’s not installed:

>>> model.export_dagmc_h5m_file(h5m_backend="pymoab")
PyMoabNotFoundError: pymoab is not installed. pymoab/MOAB is not available
on PyPI so it cannot be included as a dependency of cad-to-dagmc.

You can install pymoab via one of these methods:
  1. From conda-forge: conda install -c conda-forge moab
  2. From extra index: pip install --extra-index-url https://shimwell.github.io/wheels moab
  3. From source: https://bitbucket.org/fathomteam/moab

Alternatively, use the h5py backend (the default) which does not require pymoab:
  export_dagmc_h5m_file(..., h5m_backend='h5py')

Comparing Backends#

import cadquery as cq
from cad_to_dagmc import CadToDagmc

sphere1 = cq.Workplane().sphere(5)
sphere2 = cq.Workplane().moveTo(10, 0).sphere(2)

assembly = cq.Assembly()
assembly.add(sphere1)
assembly.add(sphere2)

model = CadToDagmc()
model.add_cadquery_object(cadquery_object=assembly, material_tags=["mat1", "mat2"])

# Export using h5py backend
model.export_dagmc_h5m_file(
    filename="dagmc_h5py.h5m",
    h5m_backend="h5py",
)
print("Created dagmc_h5py.h5m using h5py backend")

# Export using pymoab backend (requires MOAB)
model.export_dagmc_h5m_file(
    filename="dagmc_pymoab.h5m",
    h5m_backend="pymoab",
)
print("Created dagmc_pymoab.h5m using pymoab backend")

File Compatibility#

Both backends produce semantically equivalent h5m files that work with DAGMC. The files may not be byte-for-byte identical due to:

Different HDF5 compression settings
Different internal data ordering
Different metadata

However, both will work correctly with:

OpenMC with DAGMC
MCNP with DAGMC
Other DAGMC-enabled transport codes

Comparison Table#

Feature	h5py Backend	pymoab Backend
Installation	Included with pip install	Requires separate MOAB install
PyPI availability	Yes	No (conda or source)
DAGMC compatibility	Yes	Yes
File size	Similar	Similar
Performance	Similar	Similar